EP0064295B1 - Verfahren zur Verbesserung des Korrosionswiderstandes von durch chemische Umwandlung beschichtetem Aluminium - Google Patents

Verfahren zur Verbesserung des Korrosionswiderstandes von durch chemische Umwandlung beschichtetem Aluminium Download PDF

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Publication number
EP0064295B1
EP0064295B1 EP19820103813 EP82103813A EP0064295B1 EP 0064295 B1 EP0064295 B1 EP 0064295B1 EP 19820103813 EP19820103813 EP 19820103813 EP 82103813 A EP82103813 A EP 82103813A EP 0064295 B1 EP0064295 B1 EP 0064295B1
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EP
European Patent Office
Prior art keywords
aluminum
solution
nitric acid
deoxidizer
chromated
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Expired
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EP19820103813
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English (en)
French (fr)
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EP0064295A1 (de
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Walter Batiuk
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Individual
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Individual
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/78—Pretreatment of the material to be coated

Definitions

  • the invention relates to a method for treating aluminum to enhance corrosion resistance of its surface, which includes deoxidizing the aluminum surface with deoxidizing means and coating the aluminum surface with a chemical conversion solution.
  • Conversion coated aluminum has been used by airframe and other manufacturers, including the military, to improve aluminum's corrosion resistance and paint adhesion properties.
  • the aluminum is deoxidized using a chromated deoxidizer, and then coated in the conversion process with a coating resistant to corrosion.
  • a stringent corrosion resistant test must be met as outlined in Military Specification MIL-C-5541 and MIL-C-81706. These tests require subjecting the finished aluminum product to a salt spray for a lengthy period of time, such as 164 to 328 hours, respectively.
  • U.S. Patent No. 3,140,203 issued to Grunwald shows one formulation of a non-chromated aluminum de-smutter.
  • Hatch in U.S. Patent No. 3,335,096, deals with a corrosion protective chemical in a closed system which protects the metal surface only while in contact with the surface.
  • Thornhill U.S. patent No. 3,340,001 teaches a composition for protecting metal from corrosion which is constantly in contact with the metal.
  • This composition contains sodium nitrite; however, it is not obvious from Thornhill that this composition would have good results when combined with chemical conversion coating.
  • Shick in U.S. Patent No. 3,433,577, uses a composition containing sodium nitrite for protecting metal parts from corrosion. The purpose of this composition is for vapor phase corrosion inhibitors during storage, but does not indicate the applicability for enhancing the corrosion resistance to salt spraying.
  • U.S. Patent No. 3,445,400 issued to Everhart contains a chromated formulation.
  • Bland et al in U.S. Patent No. 3,510,430 discloses a ferric sulfate non-chromated deoxidizer which would not satisfactorily meet salt spray tests. The nitrates in this formulation are used to prevent pitting during deoxidization.
  • U.S. Patent No. 3,802,973 teaches an oxidizing agent to uniformly etch aluminum in an alkaline solution for the purpose of hiding surface defects.
  • the present invention discloses a method for treating aluminum metal to enhance corrosion resistance of its surface, which includes deoxidizing the aluminium surface with deoxidizing means and coating the aluminium surface with a chemical conversion solution.
  • the method is characterized by exposing the aluminum surface deoxidized with said deoxidizing means to a nitrite solution having a pH below 5, before conversion coating with said chemical conversion solution.
  • said deoxidizing means is a non-chromated deoxidizer, preferably said non-chromated deoxidizer is a nitric acid solution with a concentration of nitric acid of at least 10%, preferably 15% to 25%, by volume based on 10% nitric acid.
  • said non-chromated deoxidizer is a nitric acid solution with a concentration of nitric acid of 30%-50% based on commercial grade nitric acid.
  • a fluoride iron is present in said deoxidizing nitric acid solution since the fluoride iron is one of the best etching agents of unfinished aluminum metal, and can etch uniformly without pitting. Therefore it is preferred, that said nitric acid solution contains a sufficient concentration of fluoride iron to etch the aluminum surface at a rate of at least 0.00125 mm/hr (0,05 mils/hr). Supplement to the etching action of the fluoride iron, nitric acid is used to remove undissolved silicon from the aluminum surface and to prevent the formation of smut which is a redeposit of copper.
  • the treatment with the nitrite solution is especially effective if said nitrite solution is maintained at a pH of 3 or less.
  • said nitrite solution is substantially contributed by sodium nitrite, potassium nitrite and/or a mixture of sodium and potassium nitrite.
  • Vapor degreasing of the aluminum metal can be performed by standard techniques known to one skilled in the art. Similarly, alkaline cleaning and rinsing of the aluminum are steps commonly known to those skilled in the art and insure a clean aluminum surface prior to deoxidization.
  • the deoxidization step which is the next step is preferably produced with a non-chromated deoxidizer.
  • a chromated deoxidizer is undesirable from a pollution standpoint since chrome or chrome ions are toxic, and the effluent stream may be released to open bodies of water.
  • the invention can be practiced, however, using a chromated deoxidizer if desired and the resulting product will readily meet the salt spray requirements of the military.
  • non-chromated deoxidizers may be used in the practice of the invention without losing the beneficial effects of treating aluminum in the manner taught by the inventor.
  • the aluminum may be immersed in the deoxidizer for approximately 5 minutes to 60 minutes or greater, depending on the difficulty of scale removal.
  • the nitric acid deoxidizer performs best when a freshly prepared solution is buffered with some dissolved metallic aluminum.
  • the aluminum is then rinsed with cold water, and a supplemental treatment, differing from the prior art, is performed to allow the final conversion coated product to pass salt spray requirements.
  • the deoxidized aluminum is rinsed with a sodium nitrite solution.
  • the aluminum may either be immersed in the solution or sprayed with the solution.
  • the sodium nitrite solution is maintained at a pH less than 5 and preferably at pH 3 or below.
  • Nitric acid can supply the necessary hydrogen ion concentration.
  • the concentration of sodium nitrite is not critical, assuming the pH is less than 5, and beneficial results can be obtained with a sodium nitrite solution of 1%.
  • a water rinse of the aluminum is performed prior to chemical conversion coating. It should also be noted that potassium nitrite may be used in place of sodium nitrite with beneficial results.
  • the chemical conversion coating can be done with any of the coating solutions known to the prior art. It has been found that Alodine 1200S, manufactured by Amchem Products, Inc., Ambler, PA, works satisfactorily.
  • the aluminum is dried by warm air.
  • the drying temperature will be in the range of 43°C (110°F) to 54°C (130°F) with an optimum at 49°C (120°F).
  • B are aluminum alloy specimens 25.4 cm by 7.6 cm (10" by 3") were used in this and all other examples. Table 1 shows the results of this series of samples which illustrates the effect of sodium nitrite treatment on the salt spray test results.
  • a partially depleted chromate-type deoxidizer was used called Am Chem 6-17, a product of Amchem Products, Inc., Ambler, PA.
  • each specimen was first cleaned using an alkaline cleaner called Turco 2623, a product of the Turco Products Division of Purex Corporation Ltd., Wilmington, California. The alkaline cleaning was conducted for 30 minutes at 60°C (140°F).
  • Table 1 shows that only the specimens which were immersed in the sodium nitrite solution passed the salt spray test, i.e. the top four specimens of Table 1 had one or more corrosion pits visible to the unaided eye, indicating failure, while the bottom four specimens had no visible corrosion pits, indicating passing results.
  • Example 2 Bare alloy specimens similar to those used in Example 1 were used in Example 2. The main difference in the two examples was the use of a non-chromated ferrous ion-type deoxidizer instead of a chromated deoxidizer.
  • the deoxidizer in this example was Isoprep 184, a product of Allied-Kelite Products Division, Los Angeles, California. It was used in a 25% concentration with an etch rate of 0.0064 mm/side/hr (0.25 mils/side/hr) maintained with hydrogen fluoride addition.
  • the sodium nitrite solution was 2.3% sodium nitrite with a pH of 3.0. This example illustrates that the only specimens which passed the salt spray test (no pits visible to the unaided eye) were those immersed in the sodium nitrite solution see Table 2).
  • Example 3 is similar to Examples 1 and 2, differing only in the deoxidizer used.
  • a nitric acid-fluoride ion type non-chromate deoxidizer was used.
  • the specimens were processed in the same manner as Examples 1 and 2.
  • the deoxidizer consisted of 40% by volume of concentrated nitric acid (technical grade) with a sufficient fluoride ion concentration to produce an etch rate of 0.0064 mm/side/hr (0.25 mils/side/hr).
  • the sodium nitrite solution was of a 2.3% concentration with a pH of 3.3.
  • the results outlined in table 3 show that the specimens passing the salt spray test were those which were immersed in sodium nitrite. The specimens which were not immersed in the sodium nitrite solution failed the test.
  • Example 4 a nitric acid-fluoride ion type non-chromated deoxidizer was used as in Example 3. Instead of immersion in a sodium nitrite solution after this processing step, a potassium nitrite solution of 2.3% concentration with a 3.0 pH was used. The results in Table 4 show that the specimens which were not immersed in the potassium nitrite solution failed to pass the salt spray test. The specimens that were immersed in potassium nitrite, however, passed the salt spray test.
  • Example 5 uses Isoprep 184, a ferrous ion-type non-chromated deoxidizer as did Example 2. Instead of sodium nitrite, potassium nitrite was used in the next processing step. The solution was of 2.3% concentration with a pH of 3.0. As seen in Table 5, the only specimens passed were those treated with potassium nitrite.

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  • Chemical & Material Sciences (AREA)
  • General Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Chemical Treatment Of Metals (AREA)

Claims (11)

1. Verfahren zur Behandlung von Aluminium zwecks Verbesserung des Korrosionswiderstandes auf der Oberfläche, welches folgende Schritte umfaßt es wird die Aluminiumoberfläche mit einem Desoxydationsmittel desoxydiert und es wird die Aluminiumoberfläche mit einer chemischen Umwandlungslösung überzogen, dadurch gekennzeichnet, daß die durch das Desoxydationsmittel desoxydierte Aluminiumoberfläche einer salpetrigsauren Lösung ausgesetzt wird, die einen pH-Wert unter 5 besitzt, bevor der Umwandlungsüberzug mit der chemischen Umwandlungslösung aufgetragen wird.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Desoxydationsmittel ein chromfreies Desoxydationsmittel ist.
3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß das chromfreie Desoxydationsmittel eine Salpetersäure-Lösung mit einer Konzentration von Saltpetersäure von wenigstens 10 Volumenprozent ist, basierend auf einer 100%-igen Salpetersäure.
4. Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß die Konzentration der Saltpetersäure 15-25 Volumenprozent beträgt.
5. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß das chromfreie Desoxydationsmittel eine Saltpetersäure-Lösung mit einer Konzentration von Saltpetersäure von 30%-50% ist, basierend auf einer kommerziellen Klasse einer Salpetersäure.
6. Verfahren nach den Ansprüchen 3,4 oder 5, dadurch gekennzeichnet, daß die Salpetersäure-Lösung eine ausreichende Konzentration von Fluorid-lonen enthält, um die Aluminiumoberfläche mit einer Geschwindigkeit von wenigstens 0,00125 mm/h (0,05 mils/h) zu ätzen.
7. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß der pH-Wert der salpetrigsauren Lösung 3 oder weniger beträgt.
8. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die salpetrigsaure Lösung im wesentlichen von Natriumnitrit gebildet ist.
9. Verfahren nach einem der vorhergehenden Ansprüche 1 bis 7, dadurch gekennzeichnet, daß die salpetrigsaure Lösung im wesentlichen von Kaliumnitrit gebildet ist.
10. Verfahren nach einem der vorhergehenden Ansprüche 1 bis 7, dadurch gekennzeichnet, daß die salpetrigsaure Lösung im wesentlichen von einer Mischung aus Natriumnitrit und Kaliumnitrit gebildet ist.
11. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die Oberfläche des Aluminiums, die mit der chemischen Umwandlungslösung überzogen wird, bei einer Temperatur zwischen 43°C (110°F) und 54°C (130°F) getrocknet wird.
EP19820103813 1981-05-04 1982-05-04 Verfahren zur Verbesserung des Korrosionswiderstandes von durch chemische Umwandlung beschichtetem Aluminium Expired EP0064295B1 (de)

Applications Claiming Priority (2)

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US26037081A 1981-05-04 1981-05-04
US260370 1981-05-04

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EP0064295A1 EP0064295A1 (de) 1982-11-10
EP0064295B1 true EP0064295B1 (de) 1985-11-21

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
ZA846933B (en) * 1983-09-16 1985-04-24 Robertson Co H H Method for providing environmentally stable aluminum surfaces for painting and adhesive bonding and product produced
JP2719612B2 (ja) * 1986-01-21 1998-02-25 ヘンケル コーポレイション アルミニウムの洗浄方法

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US3779818A (en) * 1971-04-01 1973-12-18 Bethlehem Steel Corp Method for preventing the formation of rust on the surface of coiled steel strip and an inhibitor for accomplishing same
IT1013311B (it) * 1973-06-09 1977-03-30 Daimler Benz Ag Procedimento per il miglioramento superficiale di corpi o sfere di rotolamento e o di piste di scorri mento di snodi omocinetici e di cuscinetti a rotolamento
US4098720A (en) * 1973-10-25 1978-07-04 Chemed Corporation Corrosion inhibition
JPS5629699A (en) * 1979-08-15 1981-03-25 Fuji Photo Film Co Ltd Surface roughening method by electrolysis

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EP0064295A1 (de) 1982-11-10

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